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J. Scalo, D. S. Smith, J. C. Wheeler (Univ. Texas Austin)
We have developed a Monte Carlo code for calculations of the transport of high-energy irradiation of the atmospheres of ancient and contemporary Earth and Mars by solar flares and stellar explosions. We have calculated the dose spectrum to which surface organisms would be exposed during a large solar flare. We include details of Compton scattering and X-ray photoabsorption and an approximate treatment of the redistribution of the incident energy to ultraviolet and visible radiation by secondary electron excitation of atmospheric molecules, and its transfer to the surface, using Rayleigh scattering and scaled terrestrial ozone distribution as examples of UV shields. We find that for thick atmospheres (column densities greater than about 100 g/cm2 much of the incident ionizing radiation can be redistributed to biologically and chemically important ultraviolet wavelengths, a significant fraction of which can reach the surface. This radiation will consist primarily of a large number of auroral-like emission bands. This result is relevant to Earth and early Mars. For contemporary Mars, most of the energy reaches the surface as X-rays due to Compton scattering and photoabsorption, with a characteristic spectrum that is essentially independent of atmospheric composition. We calculate the dose per unit flare energy for water and for a DNA action spectrum, and we estimate the frequency of biologically significant flares from the Sun at Mars as a function of time using present-day flare statistics and studies of solar proxies of various ages. The question of sterilization of exposed organisms at the Martian surface is discussed using lethal dose data for prokaryotic and eukaryotic terrestrial organisms.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.